Autonomous evanescent optical nanosensor

ABSTRACT

A sensor includes traps that are adjacent to a waveguide and capable of holding a contaminant for an interaction with an evanescent field surrounding the waveguide. When held in a trap, a particle of the contaminant, which may be an atom, a molecule, a virus, or a microbe, scatters light from the waveguide, and the scattered light can be measured to detect the presence or concentration of the contaminant. Holding of the particles permits sensing of the contaminant in a gas where movement of the particles might otherwise be too fast to permit measurement of the interaction with the evanescent field. The waveguide, a lighting system for the waveguide, a photosensor, and a communications interface can all be fabricated on a semiconductor die to permit fabrication of an autonomous nanosensor capable of suspension in the air or a gas being sensed.

CROSS-REFERENCE TO RELATED APPLICATION

This patent document is related to a co-filed U.S. patent applicationentitled “Passive Evanescent Optical Nanosensor,” Ser. No. 11/127,869,which is hereby incorporated by reference in its entirety.

BACKGROUND

Detection of contaminants such as pollutants, toxins, poisons, andbiological agents is critically important in many industrial, public,and private environments. Accordingly, a variety of environmentalsensors have been developed. These environmental sensors are generallylarge enough to be handheld or mounted in the areas being monitored.Unfortunately, the size and need for separate mechanical and electricalcomponents make these environmental sensors expensive when compared tothe costs of integrated circuits. Sensors that could be manufacturedusing nanotechnology could potentially reduce sensing costs and permitnew sensing capabilities, for example, for environments that aredifficult to access or that have insufficient space to accommodateconventional sensors.

Fiber-optic evanescent fluorescence sensors, for example, are a knownclass of sensors used in biomedical applications. These sensorsgenerally sense or measure the concentrations of target molecules thatare known to absorb light having a first wavelength λ and tosubsequently fluoresce by emitting light having a second wavelength λ′.Such sensors typically include an optical fiber that is inserted into aliquid containing the target molecules, while light having wavelength λis directed through the optical fiber. The target molecules that arewithin the evanescent field surrounding the optical fiber can thenabsorb light of wavelength λ from the optical fiber and subsequentlyfluoresce to emit back into the optical fiber light having wavelengthλ′. A detector coupled to the optical fiber measures the intensity ofthe light having frequency λ′, and that measurement indicates thepresence or number of target molecules within the evanescent field ofthe optical fiber.

Current evanescent fluorescence sensors have a number of drawbacks. Inparticular, such sensors are relatively large and limited to sensingtarget molecules that have suitable fluorescent properties. Further,evanescent fluorescence sensors are typically limited to sensing targetmolecules in a liquid because contaminants in a gas at room temperaturespend only a short time within the evanescent field, i.e., within adistance of about λ/4 of the optical fiber, and therefore generally moveaway from the fiber before fluorescing.

In view of the limitations of current environmental sensors, inexpensivesensors and sensing methods for detecting a variety of contaminantspecies in a gas or a liquid are needed.

SUMMARY

In accordance with an embodiment of the current invention, a sensorincludes: a waveguide; a lighting system coupled to the waveguide; atrap adjacent to the waveguide; a photosensor; and a communicationsinterface. The trap is capable of capturing and holding a targetcontaminant in an evanescent field of the waveguide, and the photosensoris positioned to detect light from the trap. The communicationsinterface can be connected to the photosensor.

Another embodiment of the invention provides a method for detecting atarget contaminant. The method includes capturing a particle of thetarget contaminant in a trap adjacent to a waveguide. When light isdirected down the waveguide, the trap that has captured a particle holdsthe particle in an evanescent field caused by the light in thewaveguide. Light that the target contaminant scatters from the waveguidecan be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a nanosensor in accordance with an embodimentof the invention including a waveguide, a lighting system, traps,photosensors, and a communications interface.

FIG. 2 illustrates the change in an output signal as traps in the sensorof FIG. 1 capture particles of a target contaminant.

FIG. 3 is a block diagram of a measurement system in accordance with anembodiment of the invention using multiple nanosensors and a centralstation.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION

In accordance with an aspect of the invention, a sensor can employ trapsthat capture and hold target contaminants within the evanescent fieldsurrounding a waveguide. The presence or number of particles of thetarget contaminants in the traps can be detected at anytime aftercapture by directing light of a desired wavelength through the waveguideand measuring light scattering from the contaminants. Such scatteringcan occur through many mechanisms including but not limited to linearand nonlinear resonance fluorescence or Raman scattering. The sensorscan be made autonomous through integration of the waveguide, the traps,a lighting system, photosensors, and a communications interface into asingle device. Further, sensors including the waveguides, the traps,photosensors, and communications interface can be integrated in or on achip using nanotechnology. Accordingly, millions of inexpensivedust-sized nanosensors (each of which could have hundred or thousands oftraps) can be dispersed within a gas and used to detect and map theconcentration and/or distribution of the target contaminants.

Reference is now made in detail to specific embodiments, whichillustrate the best mode presently contemplated by the inventors forpracticing the invention. Alternative embodiments are also brieflydescribed as applicable.

FIG. 1 shows a schematic illustration of a sensor 100 fabricated on adie 110. In an exemplary embodiment of the invention, die 110 is aprocessed semiconductor chip, e.g., a silicon or GaAs chip, having anarea of several square microns. On die 110, sensor 100 includes awaveguide 120, a collection of traps 130, a lighting system 140,photosensors 150, and a communications interface 160.

Waveguide 120 is fabricated in die 110 and has optical characteristicssuitable for guiding light or other electromagnetic radiation having awavelength that interacts with a target contaminant to be measured. Inan exemplary embodiment of the invention, waveguide 120 is a channel ofdielectric material such as silica, silicon dioxide, or lithium niobatethat conventional processing techniques have deposited or grown on die110. Alternatively, waveguide 120 can be a line defect in a photoniccrystal formed in die 110. For a photonic crystal, holes or variationsof the refractive index in die 110 can be formed in a pattern such thatpropagation of light having the desired wavelength is limited to adefect corresponding to waveguide 120. Such defects are generallyimplemented as a variation in the pattern of the photonic crystal.

Traps 130 are designed to trap a target species or multiple targetspecies of contaminant and are positioned to hold the target contaminantwithin a volume corresponding to an evanescent field of waveguide 120.Accordingly, sensor 100 can sense contaminants in a gas because traps130 confine the particles of the contaminant, which may otherwise befast moving when free in the surrounding gas. Confinement of theparticles may either be permanent or at least of sufficient duration forevanescent sensing. The holding of contaminants may be advantageous notonly for sensing contaminants in a gas but also for sensing contaminantsin liquid, particularly when fluorescence is sensed and the decay timefor the contaminant to fluoresce is long. Traps 130 should further besuch that traps 130 cause little or no scattering of the light fromwaveguide 120 when traps 130 are empty. In a typical application ofsensor 100, each trap 130 is a molecular trap that is designed to trapthe same target molecule. Such molecular traps are well known in theart, and some specific examples of suitable traps are described furtherbelow.

Lighting system 140 directs light or other electromagnetic radiation forpropagation through waveguide 120. Lighting system 140 is preferably anactive light source such as a laser or light emitting diode (LED) thatis fabricated in and on die 110 and produces light having a wavelengththat causes a target contaminant in the evanescent field of waveguide120 to fluoresce or otherwise scatter the light. A power source (notshown), which may be, for example, a charged capacitor, an inductor or areceiver tuned to absorb power from an electromagnetic signal, or aphotovoltaic cell can be provided on die 110 to power lighting system140 and other circuit elements of sensor 100. Alternatively, lightingsystem 140 can be a passive system that directs light from thesurrounding ambient into waveguide 120. The contaminants captured intraps 130 can then interact with the evanescent field that the lightcreates around waveguide 120.

Photosensors 150 can be positioned along waveguide 120 or adjacent totraps 130 to detect light scattered or emitted by the contaminantscaptured in traps 130. In an exemplary embodiment of the invention,photosensors 150 include two banks of photodiodes that are fabricated indie 110 adjacent to waveguide 120. If desired, the surface of die 110can be contoured to increase the efficiency with which photosensors 150collect the light emitted or scattered from traps 130, and bafflesand/or optical filters (not shown) may be added to block stray light andto select a range of wavelengths for the light that photosensors 150measure. Additional sensing circuitry such as a current-to-voltagecurrent converter and/or an amplifier connected to photosensors 150 canbe fabricated in die 110 and used to produce a measurement signalindicating the total intensity of light measured. In the exemplaryembodiment of the invention, the measurement signal has an analogvoltage or current level proportional to a total current output from thephotodiodes in photosensors 150.

Communications interface 160 receives the measurement signal fromphotosensors 150 and produces an output signal that can be externallyreceived and interpreted. In an exemplary embodiment of the invention,communication circuit 160 includes a radio frequency (RF) transmitter ortransceiver that broadcasts an output signal to a remote receiver (notshown). The output signal can simply be an RF signal having an analogintensity that is proportional to the output signal from photosensors150, so that a receiver can measure the intensity of the RF signal atthe frequency used for signaling to determine the amount of contaminantcaptured in one or more nearby sensors 100. Alternatively, any desiredsignaling protocol, including but not limited to digital signalingprotocols, can convey measurement data from one or more sensors 100 tothe central receiver. The central receiver can process the signals fromthe sensors 100 and estimate the contaminant concentration.

In another alternative embodiment, communications interface 160implements an optical interface such as described in U.S. patentapplication Ser. No. 10/684,278, entitled “Photonic InterconnectSystem,” which is hereby incorporated by reference in its entirety.

Sensor 100 as described above uses traps 130 for capturing a targetcontaminant from a surrounding environment and for binding the capturedcontaminant within an evanescent field around waveguide 120. Indifferent embodiments of the invention, each captured particle of thetarget contaminant may be, for example, an atom, a molecule, a virus, ora microbe, and traps 130 generally have a chemistry or structure that issuitable and selected for capture of the target contaminant.Additionally, traps 130 must also firmly attach to the material ofwaveguide 120, e.g., to silica, or to another material in sensor 100adjacent to waveguide 120. Particular molecular groups such as chlorinederivatives of silane, are well known to bind strongly to silicondioxide and other materials. In an exemplary embodiment of theinvention, waveguide 120 is formed from silicon dioxide that isuniformly coated with a molecular group of chlorine derivatives ofsilane, which in turn irreversibly binds traps 130 to waveguide 120.

Some atomic species of environmental contaminants that often need to bemonitored include toxins such as arsenic (As) or lead (Pb), fissionablematerials such as uranium (U) or plutonium (Pu), and other radioactivematerials such as certain isotopes of strontium (Sr). A range of“host-guest” chemistries have been developed for capture of either aspecific type of atom or an atom from a specific chemical family such asthe alkali metals or the rare earth metals. These host-guest chemistriesoften discriminate among various atomic or ionic species based on thediameter of the atom or ion. Molecule cages known as carcerands orhemicarcerands, for example, can trap an atom (or a small molecule) andpermanently hold the trapped contaminant. In an embodiment of theinvention that measures or detects contaminant atoms or small molecules,traps 130 can be implemented as carcerands and hemicarcerands creating acage of the size required to trap a particle of the target contaminant.

Another type of trap 130 for atomic contaminants uses a chelatingcompound, such as the well-known bidentate molecule ethylenediamine orthe hexadentate molecule EDTA (ethylenediaminetetraacetate), which canform complexes with a target atom. Such chelates can also be bound towaveguide 120 using a chlorosilane chemistry such as mentioned above.

Chemistries that have been developed to complex many of theenvironmental pathogens or chemical agent molecules can also be used fortraps 130 in sensor 100. For example, various bioactive pathogens attackparticular molecular structures such as a protein or DNA in cells. Forthese pathogens, the specific protein or DNA strand may be used as“bait” trapping for the pathogen. Carcerands and other related systemshave also been developed for capture of specific molecules (or aspecific family of molecules) and could be used as traps 130 that bind amolecular species.

For a larger contaminant such as a virus, e.g. the polio or ebola virus,an antibody for the virus can be bound to waveguide 120 as traps 130because in many cases the antibody contains a protein that bindsspecifically to the external coating of the virus particle.Alternatively, traps 130 could include a suitable protein from theantibody or any other type of molecule designed to recognize and bind toa particular virus or include a type of bait that resembles the lipidlayer of a cell that attracts the virus.

Finally, sensor 100 could successfully detect microbes using entitiessuch as anthrax spores or other bacterial agents attached to waveguide120 as traps 130. Alternatively, the cell wall or coatings of microbecontaminants can be recognized or bound using specific proteins orsugars.

In order to achieve a high level of certainty in detection, sensor 100may to use several different types of traps 130 to recognize and bindthe same contaminant on either the same or different waveguides 120.With separate waveguides 120 using different traps 130, a coincidence ofseparate measurement signals could provide confirmation of trapping ofthe target contaminant and minimize spurious signals or false positives.

FIG. 2 illustrates the typical behavior of the measurement signal forsensor 100 as a function of time. Ideally, traps 130 are protected fromcontaminants until a test start time (t=0) or are activated chemicallyor electromagnetically at the test start time. At a subsequent time T1,a first of the traps 130 captures a target contaminant at which pointthe scattered light measured by photosensors 150 jumps. At subsequenttimes T2, T3, T4, T5, and T6, other traps 130 capture additionalparticles of the target contaminant, causing discrete increases in thescattered light, until all of the traps 130 are filled. The jumps in themeasurement signal identify when the target contaminant is detected.Further, the mean rate at which the measurement signal increasesindicates the concentration of the target contaminant since highercontaminant concentrations will cause higher rates of capture.Accordingly, with appropriate calibration or analysis, the rate ofsignal increases or the capture rate can be used to measure theconcentration of the target contaminant.

FIG. 3 illustrates a system 300 for using sensors 100 to measure thepresence, the concentration, and/or the distribution of a targetcontaminant. For system 300, a collection of sensors 100, which arenanosensors that are sufficiently small to float in air, are releasedinto an environment to be tested for a target contaminant. Sensors 100may, for example, be released into a ventilation system in a building.The number of sensors 100 used will generally depend on the expecteddensity of the target contaminant, the size of the environment tested,and how sensitive the environment is to the dust that discarded sensors100 create when testing is complete. Traps in each sensor 100distributed into the environment capture particles of the contaminant assensors 100 move through the environment.

A central station 310 can be moved to any location where sensors 100 arepresent. In the illustrated embodiment, central station 310 includes atransmitter that transmits a signal to activate one or more sensors 100.The transmitted activation signal may, for example, be a radio ormicrowave signal that a communications interface 160 receives andconverts to power that operates lighting system 140 and other circuitelements in sensors 100. If desired, the activation signal can bedirectional or limited in range so that only sensors 100 in a particulararea are activated.

The activated sensors 100 transmit back measurement signals indicatingthe number of captured contaminants in the activated sensors 100.Central station 310 can continue monitoring or polling of themeasurement signals from sensors 100 over a period of time sufficient todetermine rates of increase of the measurement signals and from therates of increase determine the concentrations of the contaminant in thespecific areas containing the activated sensors. Information concerningthe presence, the concentration, and the distribution of the targetcontaminant or contaminants can thus be obtained.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. For example,although the above embodiments often describe use of light and lightsources, it should be understood that embodiments of the invention arenot limited to use of visible light but more generally can employ otherwavelengths of electromagnetic radiation that provide an evanescentfield suitable for detection of target contaminants. Various otheradaptations and combinations of features of the embodiments disclosedare within the scope of the invention as defined by the followingclaims.

1. A sensor comprising: a waveguide; a lighting system coupled to thewaveguide; a trap adjacent to the waveguide, wherein the trap comprisesa molecular cage that is capable of capturing and holding a targetcontaminant in an evanescent field around the waveguide; a photosensorpositioned to detect light from the trap; a communications interfaceconnected to the photosensor; wherein the waveguide, the lightingsystem, the photosensor, and the communications interface are integratedon a semiconductor die.
 2. The sensor of claim 1, wherein the waveguidecomprises a material selected from the group consisting of silica andlithium niobate.
 3. The sensor of claim 1, wherein the lighting systemcomprises a laser integrated on the semiconductor die.
 4. The sensor ofclaim 1, wherein the trap is attached to the waveguide.
 5. The sensor ofclaim 1, wherein the communications interface comprises the RFtransceiver.
 6. The sensor of claim 1, further comprising a plurality oftraps that are adjacent to the waveguide and capable of capturing andholding the target contaminant in the evanescent field around thewaveguide.
 7. The sensor of claim 6, wherein the communicationsinterface broadcasts an output signal indicating a number of the trapsthat are holding the target contaminant.
 8. The sensor of claim 1,wherein the sensor operates to sense the target contaminant in a gas. 9.The sensor of claim 1, wherein the sensor has a size such that thesensor can float in air while sensing for the target contaminant in theair.
 10. The sensor of claim 1, wherein the molecular cage is selectedfrom a group consisting of a carcerand and a hemicarcerand.
 11. A methodfor detecting a target contaminant, comprising: directing light througha waveguide using a lighting system; capturing the target contaminant ina trap adjacent to the waveguide, wherein the trap comprises a molecularcage that holds the target contaminant in an evanescent field of thelight in the waveguide; measuring light that the target contaminant inthe trap scatters from the waveguide using a photosensor; wherein thewaveguide, the lighting system, and the photosensor are integrated on asemiconductor die.
 12. The method of claim 11, further comprisingproviding an integrated nanosensor that includes: the waveguide and aplurality of traps.
 13. The method of claim 12, further comprisingtransmitting an output signal from the nanosensor, wherein the outputsignal indicates a total intensity of light scattered from thewaveguide, at the traps.
 14. The method of claim 13, further comprisingdetermining a concentration of the target contaminant from a rate ofchange in the total intensity of the light scattered at the traps. 15.The method of claim 13, wherein the light scattered at the traps has awavelength that is the same as a wavelength of the light directedthrough the waveguide.
 16. The method of claim 13, wherein the lightscattered at the traps has a wavelength that differs from a wavelengthof light directed through the waveguide.
 17. The method of claim 11,wherein capturing the target contaminant comprises exposing the trap toa gas containing the target contaminant, wherein the trap captures thetarget contaminant from the gas.
 18. The method of claim 12, whereincapturing the target contaminant comprises permitting the nanosensor tofloat in air, wherein the traps capture the target contaminant from theair.
 19. The method of claim 12, wherein the molecular cage is selectedfrom a group consisting of a carcerand and a hemicarcerand.